1. Field of the Invention
The present invention relates generally to a system and method for reducing undesired radiation generated from LSIs. More specifically, the invention relates to an undesired radiation reducing system and method for use in automated design for LSIs.
2. Description of Related Arts
As undesired radiation (which will be also hereinafter referred to as a "noise") generated from LSIs, the following three kinds of noises shown in FIG. 7 are typically known.
(1) Power Source Noise: Swing in potential of power source lines 41a, 41b due to power consumption in an LSI circuit 40. PA0 (2) I/O Noise: Swing in potential of power source lines 41a, 41b propagated to an output signal of an input-output buffer 44. PA0 (3) Electromagnetic Radiation Noise (Radiation Noise): Electromagnetic wave radiated from the whole surface of a chip 40.
Main noise sources are circuit elements relating to clock signals (which will be also hereinafter referred to as "clock cells"), such as a flip-flop 42 or a clock buffer. Since the clock signals are synchronism with each other, currents flow simultaneously through the clock cells. Since an LSI has numerous clock cells, the total current is great. Therefore, a great number of charges are drawn out of power source lines in a short time. In order to compensate the charges drawn out of the power source lines, it is required to supply with charges from the outside of the circuit. The flow of the supplied charges serves as a power source noise observed outside the LSI.
As a method for reducing a power source noise, there is known a method for adding a capacitor to a power source line in the vicinity of a noise source. The reason why this method can reduce the power source noise is as follows. The added capacitor serves as a charge storage area. Thus, a part or a large part of current to be consumed by the noise source can be temporarily supplied from the added capacitor. Therefore, it is not required to supply charges from the outside of the LSI in a short time, so that the power source noise is reduced.
However, it is generally difficult to determine the dimension of the capacitor to be added. If the capacitor is inadvertently added, there is a possibility that the added capacitor is combined with an inductance in the circuit to cause resonance (a state of a very large noise). In addition, an addable capacitor is restricted for the following two reasons.
(1) Holding of Reliability of LSI
The capacitor in the LSI is formed of an oxide film. This oxide film is very thin, so that it is breakable. Therefore, as the number of capacitors increases, the area of the oxide films increases, so that there is a higher probability that the LSI will fail.
(2) Constraint on Available Capacitor
Circuit elements are spread all over the interior of an LSI in order to increase the degree of integration of the LSI. Thus, the area occupied by the capacitor in the vicinity of a certain noise source is restricted.
It is required to obey the aforementioned two constraints and to add capacitors so as not to cause resonance. It is very difficult to do so by the engineer's manual operation. Therefore, it is required to provide a system for automatically calculating a capacity to be added. However, such a system is not known in the present circumstances.